Optical fibers form some of the main lines through which telecommunications data is connected all over the world. An optical fiber typically includes a core region surrounded concentrically by a cladding. Some fiber designs, known as “double cladding” designs, surround the core region with an inner cladding, which is in turn surrounded by an outer cladding. The outer cladding likewise is surrounded by an external medium.
The parameters in such double cladding waveguide designs usually are:
Dependent Parameters
                MFD operational mode field diameter        λ operational wavelength        λc second mode cutoff wavelengthIndependent Parameters        rco core radius generated from a single effective (refractive index) step approximation of the core region        ric inner cladding radius        W inner cladding width (ric−rco) generated from a single effective step approximation of the inner cladding region        roc outer cladding radius        nco core refractive index generated from a single effective step approximation of the core region        nic inner cladding refractive index generated from a single step approximation of the inner cladding region        noc outer cladding refractive index        next external medium refractive index+Δ=(nco−noc)/noc−Δ=(nic−noc)/nocΔTot=|(+Δ)|+|(−Δ)|        
Early large-scale commercial systems were designed to operate at an operational wavelength λ of about 1300 nm, a region of relatively low optical absorption loss and very low chromatic dispersion for silica fibers. In addition, intrinsic optical absorption losses in silica fibers were known to be even lower at 1550 nm.
Fiber suppliers began making telecommunications optical fibers that could operate at either 1300 nm or 1550 nm, such as Corning SMF-28™ single mode fiber. This fiber has a typical core diameter of 8.2 micrometers and a MFD of about 9–10 microns in the window from 1300 nm to 1550 nm. This fiber is known as a “matched clad” design with an effective step index core having a (normalized or relative) core refractive index above the outer cladding (+Δ=(nco−noc)/noc) of about 0.0035.
Early fiber optic telecommunication systems involved “long haul” applications from one telephone company central office to another. Long haul telecommunications fibers are typically kept relatively straight in large multi-fiber cables, and are thus protected from bending losses of light due to exceeding the critical bend radius of the fiber design (typically in the range of 25 mm to 12.5 mm).
A recent trend had been to extend fiber optics outward from the central offices, providing “fiber to the campus” and “fiber to the desktop” in commercial buildings, and “fiber to the neighborhood” and eventually “fiber to the home” in residential areas, sometimes referred to as “Fiber-To-The-X,” or “FTTX.” One conventional example for premises applications is the Volition™ VF-45 fiber optic connector and premise “wiring” system, manufactured by 3M Company, of St. Paul, Minn., as shown in several patents, including U.S. Pat. No. 5,757,997.
To minimize optical losses in connectors such as these, the optical fibers are designed to operate at the same wavelength and have approximately the same mode field diameter (MFD) at that wavelength. For such connectors, it is not practical to adjust the MFD of the two fiber ends (for duplex applications) by high temperature diffusion of core dopants, as can be done when fusion splicing two optical fibers for long haul cables (see, e.g., EP 1094346 A1).
As a light signal travels in an optical fiber the signal is attenuated, due to both material effects and waveguide effects. Waveguide effects include two categories of optical bending loss, microbending and macrobending losses. Macrobending loss occurs when a length of fiber is bent into a curve such that some light is radiated out of the core into the cladding of the fiber and lost. Microbending losses result from concentrated pressure or stresses exerted on the surface of the fiber. Microbending loss occurs when the fiber is exposed to localized pressures and stress points as, for example, if the fiber is pressed against a rough textured surface (such as sandpaper). When the outer surface of the fiber is pressed against the raised points, a coating that is too hard may transfer these stresses to the core, causing scattering losses. Microbend losses are usually negligible for short lengths of fiber.
Such stresses may be reduced by providing a relatively soft, low-modulus inner coating on the surface of the glass fiber. However, usually such coatings are removed from the fiber end in order to accurately align a single mode fiber with another fiber in a connector. The stripped fiber ends are then susceptible to breakage from abrasion and moisture.
A solution to this problem is a fiber having a glass core, glass cladding, polymer cladding construction (referred to herein as a “GGP” fiber), as described in U.S. Pat. No. RE 36,146, which is hereby incorporated by reference. The RE 36,146 patent describes several polymer coatings and other coating materials that can be used in manufacturing GGP fibers. Other polymer coatings and/or coating materials are described in U.S. patent Publication No. 2003-0099451 A1, now allowed; U.S. Pat. No. 6,587,628, U.S. Pat. No. 5,644,670; and U.S. Pat. No. 6,269,210 (all of which are hereby incorporated by reference).
These polymer coatings typically have a Shore D hardness of about 55 or more, or a Young's Modulus of from 50 kg/mm2 to 250 kg/mm2 at room temperature, and these coatings can adhere tightly to the outermost glass surface of the optical fiber. They are exemplarily applied to an optical fiber such that their outer surface is sufficiently concentric with the core of the optical fiber that when a GGP fiber is placed in a typical fiber optic mechanical connector and optically connected to a second fiber, the optical loss is not significantly greater than for a similar connection using an uncoated fiber having the same outer diameter as the GGP optical fiber.
In addition, the glass portion of the GGP fiber can be smaller than the standard 125 micrometer outside diameter, and an adherent, very concentric, and relatively hard polymer layer (coating) is added to bring the fiber diameter up to the standard 125 micrometer diameter while maintaining concentricity for connectorization. The construction is cabled within a low-modulus coating to minimize microbending losses, but when the low modulus coating is stripped off for connectorization the outer glass surface of the fiber is not exposed or damaged.
Among the optical fiber applications with the most severe bending loss requirements have been the fiber optic guided missile (FOG-M) and tethered weapons applications for the military. Designs for fibers used in tethered weapon applications have concentrated on keeping the light signals very tightly confined in the fiber core, by designing fibers with small MFD (˜4–7 micrometers at 1550 nm). Some designs include a depressed refractive index well around the core (so called “W” fibers) that provide for a broader range of operating wavelengths. The high matched clad index design may also provide reasonable bend tolerance if designed to operate at a single wavelength. Examples of depressed well, small MFD fibers are described in U.S. Pat. Nos. 4,838,643, and 5,032,001.
Although these fibers provide low bend loss, their small MFDs make them unsuitable for connectorization to the low cost, large (>8.0 microns) MFD telecom fibers. Multi-wavelength versions of these fibers have the smallest MFDs and therefore the largest MFD mismatches and associated connector losses making them impractical for premise applications. These fibers can only be fusion spliced or thermally treated to eliminate the MFD mismatch, which are not practical procedures for the multiple plug-in/disconnect applications.
In general, fiber designs with smaller MFDs have a higher numerical aperture (NA) at a given wavelength, since both parameters indicate a more tightly confined optical mode, which will be less affected by macrobending or other external influences. The relationship between MFD, macrobending loss, and second mode cutoff wavelength is discussed in U.S. Pat. Nos. 5,608,832 and 5,278,931, and references therein. Further discussion of the relationship between MFD, macrobending loss, and second mode cutoff wavelength is discussed in the commonly owned, co-pending U.S. patent Publication No. 2003-0099451 A1 (incorporated by reference herein in its entirety).